(a) Field of the Invention
This invention relates to an acoustic resistor used in electroacoustic transducers such as mircrophones, loudspeakers, etc. and particularly to an acoustic resistor which is placed at a tone aperture portion of an electroacoustic transducer and serves to provide the acoustic impedance of the aperture portion with a proper acoustic resistance component. The invention relates also to a method for manufacturing such acoustic resistor.
(b) Description of the Prior Art
FIG. 1 shows the structure of a unidirectional dynamic microphone as an example of the electroacoustic transducer. In this figure, a magnet 1 is mounted on a bottom yoke 2 and they are entirely housed in a centering 3. A lateral side of the centering 3 is covered with a top yoke 4. A pole piece 5 located atop the magnet 1 is provided in such a manner that it projects from the centering 3. Above the pole piece 5 is located a diaphragm 7 provided with a voice coil 6. Above the diaphragm 7 is mounted a cover 8 serving also as a high frequency compensating adapter. A terminal board 10 is fixed to the bottom yoke 2 by a screw 9. Under the bottom yoke 2 is provided a cavity case 14.
In the above structure, acoustic resistors 11 are bonded to the cavity case 14 and top yoke 4 so as to cover their respective circular and rectangular tone apertures 12. The acoustic resistors 11 are made of felt, fabric materials or metallic nets of a large number of meshes, or the lake. Conventionally these materials were cut into a desired shape and size and bonded as such to cover the tone apertures 12. Thus, since the whole tone resistor 11 is made of a material offering an acoustic resistance, portions (indicated by a mesh) of the material constituting the acoustic resistor 11 where a bonding agent 13 has penetrated shown in FIG. 2 become acoustically opaque (i.e., unable to offer an acoustic resistance), thus altering the value of the acousitc resistance component originally intended to be afforded. Further, the area and volume of the portion which apparently contributes to the acoustic resistance vary according to the state of adhesion so that the same acoustic resistors 11 could afford different acoustic impedances after adhesion to peripheral portions of the tone apertures. Accordingly, precise adhesion is required and, therefore, the work efficiency is reduced. Further, since the whole acoustic resistor 11 is made of a material offering an acoustic resistance, the size of the tone aperture 12 constitutes a factor to determine the acoustic impedance so that errors in forming of the tone aperture portion result in variation of the acoustic impedance thereof even if uniformity in adhesion among the acoustic resistors is achieved.